Intervertebral implant

ABSTRACT

In an intervertebral implant, having an upper part that has a support face for a vertebra and a lower part that has a support face for an adjacent vertebra, on each of which parts engagement elements, which are accessible from one side of the intervertebral implant, for a manipulation instrument are disposed, in order to minimize the structural height of the intervertebral implant upon insertion into an intervertebral space, it is proposed that the upper part and lower part each have protrusions and recesses aimed at the respectively other part, which are offset laterally from one another in such a way that when the upper part has been brought close to the lower part they mesh with one another; and that the engagement elements on the upper part and on the lower part are each disposed in protrusions of these parts in such a way that the engagement elements of the upper part and lower part are located side by side and at least partly overlap in the direction of the height of the intervertebral implant.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 10/998,951, now pending, which is in turn a continuation of U.S. application Ser. No. 10/018,402, now U.S. Pat. No. 6,936,071, which was a national stage application based on International Application Serial No. PCT/EP99/04628, filed Jul. 2, 1999.

BACKGROUND OF THE INVENTION

The invention relates to an intervertebral implant, having an upper part that has a support face for a vertebra and a lower part that has a support face for an adjacent vertebra, on each of which parts engagement elements, which are accessible from one side of the intervertebral implant, for a manipulation instrument are disposed, in order to minimize the structural height of the intervertebral implant upon insertion into an intervertebral space.

One such intervertebral implant is known for instance from U.S. Pat. No. 5,314,477. This intervertebral implant is used to replace a disk removed from the intervertebral space, and accordingly the intervertebral implant must have a relatively low structural height, since it has to fit into the gap between vertebrae. This is particularly difficult if an additional pivot insert is also embedded between the upper part and the lower part, as is the case in the known intervertebral implant of U.S. Pat. No. 5,314,477.

But even in two-piece intervertebral implants, difficulties also arise, especially if the implants also have pins and other protrusions on their support faces that are intended for anchoring the intervertebral implant in the bone. Often, these parts can be inserted only by widening the intervertebral space greatly. Not only is this difficult, but it also presents the risk of injuries.

Since the intervertebral space has a relatively low height, it is also difficult for engagement elements that a manipulation instrument can engage to be secured to both parts of the intervertebral implant. It is conventional to have such manipulation instruments engage the upper part and the lower part separately, for instance by means of pins that are inserted into bores on the upper part and lower part, so that with the manipulation instrument, the two parts of the intervertebral implant can be inserted into the intervertebral space and can optionally also be varied in terms of their spacing from one another, thereby allowing a certain spreading open of the intervertebral space. In this respect, reference is made to the pincerlike manipulation instrument of U.S. Pat. No. 5,314,477.

Because of the strong forces, it is necessary to provide a certain structural height for the engagement elements; for instance, the receiving bores must have a certain diameter. This dictates a minimum structural height for the upper part and for the lower part, and in conventional intervertebral implants, the structural heights of the upper part and lower part are thus added together, so that even if the upper and lower parts rest directly on one another, a relatively great structural height of the intervertebral implant is still unavoidable.

SUMMARY OF THE INVENTION

It is the object of the invention to embody an intervertebral implant of this generic type in such a way that the minimum structural height is reduced, to make it easier to insert the intervertebral implant into the intervertebral space.

In an intervertebral implant of the type described at the outset, this object is attained in accordance with the invention in that it is proposed that the upper part and lower part each have protrusions and recesses aimed at the respectively other part, which are offset laterally from one another in such a way that when the upper part has been brought close to the lower part they mesh with one another; and that the engagement elements on the upper part and on the lower part are each disposed in protrusions of these parts in such a way that the engagement elements of the upper part and lower part are located side by side and at least partly overlap in the direction of the height of the intervertebral implant.

In such an embodiment, a minimal structural height of the two intervertebral implant parts resting on one another can be attained, since the engagement elements, which cannot fall below a minimal structural height, are each disposed in protrusions of the upper part and lower part, or in other words in the parts of the upper part and lower part that have the greatest structural height. These regions of great structural height are embodied as protrusions, next to which are respective recesses, into which the protrusions of the respectively other part can dip. As a result, on the one hand, the engagement elements for the manipulation instruments are located side by side, and on the other, they can at least partly overlap, so that the total structural height of the parts resting on one another of the intervertebral implant can be reduced markedly compared to conventional intervertebral implants. The result is accordingly an internested arrangement of the upper and lower parts, with maximal exploitation of the available material height.

It is favorable if the engagement elements are insertion openings for pinlike retaining elements of a manipulation instrument; because of the described construction, these insertion openings can have a relatively large diameter and can thus receive strong retaining pins, and nevertheless a relatively low structural height of the intervertebral implant with parts resting directly on one another is obtained.

It is advantageous if the insertion openings extend substantially parallel to the support faces; once again, this prevents an increase in the structural height of the intervertebral implant parts.

In a preferred embodiment, it is provided that the lower part has a central indentation, opposite the lower support face, which indentation is surrounded by a U-shaped edge. Thus with the lower part and upper part resting directly on one another, the indentation serves to receive a protrusion on the upper part.

It is advantageous if the upper part has a central protrusion that fits substantially in complimentary fashion into the indentation; that is, the total volume of the indentation is utilized for the protrusion.

It is also advantageous if the engagement elements of the lower part are disposed on the two ends of the U-shaped edge, or in other words are located on the outside.

Conversely, the engagement elements of the upper part can be disposed on the central protrusion of the upper part, or in other words are located farther inward than the engagement elements of the upper part.

In particular, the engagement elements of the upper part can be disposed near the lateral edges of the central protrusion, so that for the upper part as well, the spacing of the engagement elements can be selected to be relatively great; as a result, both the upper part and the lower part can be reliably secured against skewing.

It should already be noted here that the words “lower part” and “upper part” do not necessarily say anything about the installed position of the intervertebral implant in the spinal column; the part called the “lower part” could in fact be above in the spinal column. Therefore, these parts may also be referred to as first and second parts having outer and inner surfaces. What is essential is merely that the upper part and lower part define the intervertebral implant on opposite sides of the implant.

It is especially advantageous if the upper part and/or the lower part is embodied in substantially platelike fashion; these parts naturally, in accordance with the design of the invention, have protrusions and recesses that are oriented toward the respectively other part. The platelike embodiment, however, leads as a whole to a very low structural height of the intervertebral implant.

In a preferred embodiment, the lower part and the upper part each have a respective receptacle for a pivot insert. This pivot insert, which is placed between the upper part and lower part after the insertion of the intervertebral implant, supports the upper part and lower part against one another; it takes on a resilient function, for instance, and furthermore leads to a certain pivotability of the two parts of an intervertebral implant relative to one another, so that a pivotability of the adjacent vertebra is thus attainable as well.

In particular, it is advantageous if the pivot insert has at least one spherical support face, which engages the correspondingly spherically shaped receptacle.

It is favorable if the spherical receptacle is disposed in the central protrusion of the upper part.

It is also advantageous if the central indentation of the lower part forms the receptacle for the pivot insert.

According to a preferred embodiment of the invention, it is provided that the pivot insert can be inserted from the side into the receptacle, which has the engagement elements for a manipulation instrument. This is the side from which the upper part and lower part are introduced into the intervertebral space, and it is also from this side that the pivot insert can then be thrust between the already-inserted parts of the intervertebral implant.

It is favorable if the pivot insert is insertable into the receptacle along a guide.

In that the insert as well is preferably embodied substantially in platelike fashion.

An especially favorable design is obtained if the insert substantially completely fills up the central receptacle and with its spherical support face protrudes from the receptacle.

BRIEF DESCRIPTION OF THE DRAWINGS

The ensuing description of preferred embodiments of the invention serves in conjunction with the drawing to provide further explanation. Shown are:

FIG. 1: a perspective exploded view of an intervertebral implant with an upper part, a lower part, and a pivot insert that can be inserted between them;

FIG. 2: a perspective exploded view of the upper part and the lower part of the intervertebral implant, without an inserted pivot insert;

FIG. 3: a view similar to FIG. 2 with the pivot insert inserted into the lower part;

FIG. 4: a perspective view of the upper part and the lower part of the intervertebral implant with maximum mutual proximity;

FIG. 5: a front view of the intervertebral implant of FIG. 4;

FIG. 6: a perspective view of the intervertebral implant with the pivot insert inserted; and

FIG. 7: a cross-sectional view of the intervertebral implant of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The intervertebral implant 1 shown in the drawing includes three parts, namely a platelike upper part 2, a platelike lower part 3, and a substantially platelike pivot insert 4. In plan view, the implant, although having rounded corners, is generally rectangular.

The upper part 2 is embodied flat on its top, thus creating a support face 5, on which various kinds of protrusions 6, 7 are disposed which serve the purpose of anchoring the upper part 2 in a vertebra that rests, with its end face toward an intervertebral space, on the support face 5. As is most evident from FIG. 5, the height of each smaller protrusion 7 is not greater than its width.

The upper part 2 is substantially rectangular in cross section; in the exemplary embodiment shown, a longitudinal edge 8 curves outward.

On the two short sides of this rectangle, the thickness of the platelike upper part 2 is less than in the central region, so that along the short sides of the upper part 2, downward-pointing recesses 9 each extending parallel to these edges are formed that are open toward the outside. The central region of the upper part 2 is located between the two recesses 9 and thus has a greater thickness or height and thus forms a downward-pointing protrusion 10 embodied between the two recesses 9. This protrusion is defined by an underside 11, which extends substantially parallel to the support face 5 and in which there is a spherical indentation 12, which forms a bearing plate for the pivot insert 4.

The lower part 3 of the intervertebral implant 1 is also platelike in embodiment and on its underside has a flat support face 13 with protrusions 14 and 15, which correspond to the protrusions 6 and 7 of the support face 5. On the side remote from the support face 13, the thickness of the lower part 3 is less in the central region than in an outer region. This outer region of greater thickness has the form of a U, with two parallel legs 16, 17, which extend parallel to the short edges of the lower part 3, which in cross section is embodied similarly to the upper part 2, and with a crosspiece 18 that connects the two legs 16 and 17 on one end. The region enclosed by the legs 16 and 17 and the crosspiece 18 forms a central indentation 19, whose area is substantially equivalent to the area of the central protrusion 10 of the upper part 2, while the disposition and length of the legs 16 and 17 correspond essentially to the disposition and length of the recesses 9 on the upper part 2. As a result, it is possible to place the upper 2 and lower part 3 on one another in such a way that the central protrusion 10 of the upper 2 dips into the central indentation 19, while the legs 16 and 17 of the lower part 3 dip into the recesses 9 of the upper part 2 (FIG. 4); in this position, the upper part 2 and lower part 3 have maximum proximity to one another and a minimal structural height.

The dimensions are selected such that the various recesses are essentially filled completely by the protrusions dipping into them.

Blind bores 20 and 21 are machined into the two legs 16 and 17 of the lower part 3, extending parallel to these legs 16, 17 from their free ends; the diameter of these bores is relatively great in proportion to the height of the legs 16, 17, and this diameter is in fact greater than the thickness or height of the lower part 3 in the region of the central indentation 19.

Blind bores 22 and 23, which extend parallel to the blind bores 20 and 21 in the lower part 3, are machined into the central protrusion 10 of the upper part 2, in the vicinity of its side edges. These blind bores 22 and 23 again have a relatively great diameter, which corresponds to a substantial portion of the height of the protrusion 10 and is greater than the thickness of the upper part 2 in the region of the recesses 9.

When the upper part 2 and lower part 3 rest tightly against one another in the manner described, the blind bores 20 and 21 of the lower part 3 and the blind bores 22 and 23 of the upper part 2 overlap at least partly in the direction of the height of the intervertebral implant 1, as is clearly shown in FIGS. 4 and 5.

The blind bores 20, 21, 22 and 23 serve as receptacles for pinlike extensions of a manipulation instrument, not shown in the drawing, and thus form engagement elements for this manipulation instrument, which in this way separately engages the upper part 2 and the lower part 3. With this manipulation instrument, it is possible to introduce the upper part 2 and the lower part 3 of the intervertebral implant 1 into an intervertebral space; the very low structural height of the intervertebral implant 1 facilitates this introduction, which can be done essentially without major widening of the intervertebral space.

After the introduction of the upper part 2 and lower part 3 in this way, the two parts of the intervertebral implant 1 can be spread apart; that is, their spacing is increased, for instance with the aid of the manipulation instrument that is holding the upper 2 and the lower part 3. Thus, in accordance with the method of operation, the upper and lower parts (or the first and second parts) are first inserted together into the intervertebral space, after which these parts are separated from each other and the insert is inserted between them. Thereafter, these parts are allowed to come together towards each other to engage the insert.

In this spread-open position of the upper part 2 and lower part 3, it is possible to thrust the pivot insert 4 between the upper part 2 and the lower part 3.

This pivot insert is constructed essentially in the shape of a plate, which has a flat underside 24 and a spherically upward-curved top side 25. The outer dimensions of the platelike pivot insert correspond to those of the central indentation 19 in the lower part 3, so that the pivot insert 4 can be thrust into this indentation, filling it up, specifically from the side toward which the blind bores 20, 21, 22, 23 open. Guide strips 26 on the side edges of the pivot insert 4 engage corresponding guide grooves 27 in the legs 16, 17, so that an insertion guide for the pivot insert 4 is formed that fixes it in the lower part 3 after its insertion. The inserted pivot insert 4, after insertion, fills up the indentation 19 and protrudes with its spherically curved top side 25 upward past the top side of the lower part 3; the spherical top side 25 dips in complimentary fashion into the spherically curved indentation 12 on the underside of the protrusion 10, where with the upper part 2 it forms a ball joint, which enables a certain pivotability of the upper part 2 relative to the lower part 3 (FIG. 7).

The pivot insert 4 can have a detent protrusion 28 on its flat underside 24; when the pivot insert 4 is inserted into the lower part 3, this protrusion locks elastically into a detent recess 29 that is located on the bottom of the indentation 19; as a result, the pivot insert 4 is also fixed in the insertion direction in the indentation 19.

The upper part 2 and lower part 3 are preferably made of physiologically safe metal, such as titanium, while the pivot insert 4 preferably comprises a likewise physiologically safe plastic material, such as polyethylene. These support faces 5 and 13 can be embodied in an especially bone-compatible way; for instance, this surface can be roughened by a coating, so that optimal anchoring to the adjacent bone material is obtained.

The invention may also be described as follows, which description is the full equivalent of the preceding discussion. An upper part 2 has an upper surface 5 for engaging a vertebrae and a lower surface which comprises a downward pointing protrusion 10 between side recesses 9 and a rounded portion, preferably in the form of a concave spherical indentation 12. A lower part 3 has a lower surface 13 for engaging a vertebrae. A pivot insert 4, when joined to the lower part 3, as shown for example in FIG. 3, provides a convex upper surface portion 25, preferably spherical, in operational engagement with the rounded portion 12 of the upper part.

The lower part 3 and pivot insert 4 may, taken together, be described as a lower part formed in two pieces, namely the elements 3 and 4, wherein the element 3 may be referred to as a lower piece and the element 4 may be referred to as an upper piece.

The upper and lower parts include on their upper surface and lower surface, respectively, protrusions 6 and 14 which may also be referred to as anchors, which anchor the upper and lower parts, respectively, into the adjacent vertebrae that form the intervertebral space and rest against the respective upper and lower surfaces.

As shown in the figures, the anchors 6 and 14 are each single anchors, preferably extending perpendicular to the respective outer surfaces from which they extend. The anchors may also be referred to as anchor portions and the plane or line along which the anchor or anchor portions extend may be referred to as an anchor line. Each of the anchors 6 and 14 has a zigzag edge which comprises a series of teeth. As best shown in the cross-sectional view of FIG. 7, anchor 6 has at least one section that is greater in height than the remainder of the upper part 2, i.e., from surface 5 to the bottom of protrusion 10, at the section of the upper part where the anchor section meets the surface of the part. Similarly, at least one section of anchor 14 is greater in height than the remainder of the lower part 3 exclusive of its convex portion, i.e., from lower surface 13 to the top of walls 16, 17 and 18. As can be seen from the perspective view of these same parts shown in FIG. 6, anchor 6 is aligned along a single anchor line across surface 5 to form a single anchor plane, this anchor plane having an anchor plane height at least a portion of which is greater than the height of the remainder of the upper part 2 at the section of the upper part where the anchor plane portion meets the surface. Alternatively, or additionally, as best shown in the front view of the parts depicted in FIG. 5, at least a portion (here, at least the facing portion) of anchor 6 is at least substantially equal in height to the overall height of the remainder of the upper part 2, i.e., from surface 5 to the bottom of protrusion 10 with its longitudinal edge 8. Furthermore, as can be seen in the perspective view of these same parts shown in FIG. 4, by virtue of the sloping edge of the anchor 6 at least a portion of the anchor plane height of anchor 6 may be less than the overall height of the remainder of the upper part 2. As also shown in the figures, in the preferred embodiment, the length of the anchors 6 and 14, i.e., in the direction from the anterior to the posterior thereof, is greater than one half of the overall dimension of its respective part from its anterior to its posterior (as defined with reference to the part's anterior posterior orientation in the body upon implantation), passing through that anchor. As also noted in the figures, the vertical height of each anchor 6 and 14 is greater than its width which is the dimension taken horizontally in FIGS. 5 or 7. Furthermore, in the illustrated embodiment as shown in FIGS. 5 and 7 the anchors 6 and 14 lie essentially in a common vertical plane in the assembled implant. As best shown in FIG. 7, pivot insert 4 enables a certain pivotability, or articulating motion, of the upper part 2 relative to the lower part 3. Midlines are lines on the outer surfaces passing essentially through the center of the outer surfaces of parts 2 and 3. Such midlines may divide the part into two separate symmetrical halves. In the illustrated embodiment, the common vertical plane of anchors 6 and 14 is located essentially at a midline of the implant which lies in an anterior to posterior plane of the implant, again as defined with reference to the intended orientation of the implant upon implantation. Each anchor 6 and 14 is elongated along the anterior to posterior midline and the width of each anchor straddles the midline.

The lower part comprises three walls including parallel side walls 16 and 17 and a rear wall 18. These walls form between them a central indentation 19 which comprises a recess with a generally flat surface. The fourth side of the recess is open. The pivot insert 4 has a detent 28 that snap-fits into a detent recess 29 formed in the generally flat surface of recess 19.

As best shown in FIGS. 2, 4 and 5, in the absence of pivot insert 4, the protrusion 10 of upper part 2 can fit down between walls 16, 17 and 18 of the lower part 2. This fitting of protrusion 10 within the recess 19, surrounded by walls 16, 17 and 18 may be referred to as “nesting” since the protrusion 10 essentially “nests” within recess 19. With the upper and lower parts in this nested condition, as shown in FIGS. 4 and 5, the combined height of the upper and lower parts 2 and 3, i.e., the height from surface 13 to surface 5, is less than the total additive height of the upper and lower parts, taken separately, i.e., less than the total of the height from surface 13 to the top of walls 16, 17 and 18 plus the height from surface 5 to the bottom of protrusion 10.

To reach its final destination within an intervertebral space, the implant must of course be moved along a path, i.e., an insertion direction from outside of the patient, into the patient, and then into the intervertebral space. In the illustrated embodiment, as described above, instruments would engage apertures 20, 21, 22 and 23 to move implant along a path in an insertion direction. The anchors 6 and 14 are parallel to this path and each defines a single anchor line parallel to this path as the implant is moved into an intervertebral space. Each anchor is thus adapted to enter a corresponding groove in the adjacent vertebra as the implant is inserted. As a point of reference, lateral planes parallel to the direction of this path pass through the outermost boundaries of the implant which, in the preferred embodiment, would be the opposed side surfaces of the parts. Each anchor 6 and 14 lies at a midline at the center of the outer surface of each part 2 and 3 in between and parallel to the lateral parallel planes. Thus, in the illustrated embodiment, the path would be parallel to the front to rear (anterior to posterior) direction, wherein, during insertion, the rear (posterior) of the implant would constitute the lead end and the front (anterior) thereof would constitute the trailing end.

Although the invention has been described in detail with respect to preferred embodiments thereof, it will be apparent that the invention is capable of numerous modifications and variations, apparent to those skilled in the art, without departing from the spirit and scope of the invention. 

1. An intervertebral implant comprising: a first part having an outer surface engaging a first vertebrae, a second part having an outer surface engaging a second vertebrae, the first and second parts being in operative engagement with each other for movement of one part relative to the other part, a single anchor extending outwardly from one of said parts, said single anchor constructed to engage a groove cut into the vertebrae which it engages to anchor that part to that vertebrae.
 2. An intervertebral implant according to claim 1, wherein the height of the single anchor is greater than its width.
 3. An intervertebral implant according to claim 2, wherein the operative engagement of the first and second parts is a universal engagement.
 4. An intervertebral implant according to claim 3, wherein the universal engagement includes a convex surface on one of the parts operatively engaged with a mating concave surface on the other part.
 5. An intervertebral implant according to claim 4, wherein one of said parts comprises two pieces, a first piece which engages the adjacent vertebrae and a second piece which engages the other part.
 6. An intervertebral implant according to claim 2, wherein the single anchor passes through a center line of the implant.
 7. An intervertebral implant according to claim 6, wherein the single anchor is located in a central anterior to posterior plane of the implant.
 8. An intervertebral implant according to claim 2, wherein both of said parts have a said single anchor, each having a height greater than its width.
 9. An intervertebral implant according to claim 8, wherein the two single anchors lie in a common plane.
 10. An intervertebral implant according to claim 9, wherein the common plane is a central anterior to posterior plane of the implant.
 11. An intervertebral implant according to claim 8, wherein the two single anchors are coplanar and vertically aligned with each other.
 12. An intervertebral implant according to claim 2, the single anchor positioned on the implant such that the single anchor engages the groove in its respective vertebrae as the implant is moved into the intervertebral space.
 13. An intervertebral implant according to claim 12, wherein both parts have a single anchor.
 14. An intervertebral implant according to claim 1, including a single anchor on both of the first and second parts.
 15. An intervertebral implant according to claim 14, wherein the two single anchors lie in a common plane.
 16. An intervertebral implant according to claim 15, wherein the common plane is a central anterior to posterior plane of the implant.
 17. An intervertebral implant according to claim 14, wherein one of the parts comprises two pieces, a first piece which includes a single anchor and a second piece operatively connected to the first piece and engaging the other part.
 18. An intervertebral implant according to claim 17, wherein the operative engagement is a universal engagement which includes a convex surface on one part and a mating concave surface on the other part, and wherein the part with the two pieces is the part which includes the convex surface.
 19. An intervertebral implant according to claim 14, wherein the two single anchors are coplanar and vertically aligned with each other.
 20. An intervertebral implant according to claim 14, wherein the operative engagement of the first and second parts is a universal engagement.
 21. An intervertebral implant according to claim 14, wherein both of the parts are generally rectangular.
 22. An intervertebral implant according to claim 21, wherein the single anchors are substantially parallel to the shorter sides of the generally rectangular shape.
 23. An intervertebral implant according to claim 14, wherein the two parts of the implant are adapted for implantation together along a path of insertion into an intervertebral space.
 24. An intervertebral implant according to claim 23, wherein the single anchors are essentially coplanar and aligned with each other, such that the single anchors of the first and second parts of the implant can be inserted along a path of insertion into respective essentially aligned grooves in the first and second vertebrae.
 25. An intervertebral implant according to claim 14, wherein the single anchor is adapted to enter a groove in the vertebrae which it is adjacent as the implant moves into the intervertebral space.
 26. An intervertebral implant according to claim 1, wherein the single anchor is located along a midline extending essentially midway between sides of its part, which sides are in planes essentially parallel to the single anchor.
 27. An intervertebral implant according to claim 26, wherein the part with the single anchor is adapted for implantation along a path of insertion into an intervertebral space.
 28. An intervertebral implant according to claim 27, wherein the overall length of the single anchor in the direction of a path of insertion of the implant is greater than one half of the overall dimension of its respective part in the direction of insertion.
 29. An intervertebral implant according to claim 27, wherein the single anchor is a single uninterrupted solid piece extending outwardly from the outer surface of its respective part.
 30. An intervertebral implant according to claim 26, wherein the first and second parts are generally rectangular.
 31. An intervertebral implant according to claim 26, wherein the height of the single anchor is greater than its width.
 32. An intervertebral implant according to claim 26, wherein the operative engagement of the first and second parts is a universal engagement.
 33. An intervertebral implant according to claim 26, wherein the height of the single anchor is greater than the height of the remainder of its part.
 34. An intervertebral implant according to claim 26, wherein the operative engagement of the two parts is a concave portion in one of the parts and a convex portion in the other of the parts, one of the two parts comprising two pieces, a first piece which includes the single anchor and a second piece operatively connected to the first piece and engaging the other part, and wherein, in the absence of the second piece, the concave portion nests into the first piece.
 35. An intervertebral implant according to claim 34, wherein the second piece snap fits into the first piece.
 36. An intervertebral implant according to claim 1, wherein the single anchor passes through the center of the outer surface of its respective part.
 37. An intervertebral implant according to claim 36, wherein the single anchor is located in a central anterior to posterior plane of the implant.
 38. An intervertebral implant according to claim 36, wherein the height of the single anchor is greater than its width.
 39. An intervertebral implant according to claim 36, the single anchor positioned on the implant such that the single anchor engages the groove in its respective vertebrae as the implant is moved into the intervertebral space.
 40. An intervertebral implant according to claim 36, wherein both parts include apertures in one end thereof for receiving an insertion instrument.
 41. An intervertebral implant according to claim 36, wherein the operative engagement of the two parts is a concave portion in one of the parts and a convex portion in the other of the parts.
 42. An intervertebral implant according to claim 41, wherein one of the two parts comprises two pieces, a first piece which includes the single anchor and a second piece operatively connected to the first piece and engaging the other part.
 43. An intervertebral implant according to claim 42, wherein, in the absence of the second piece, the concave portion nests into the first piece.
 44. An intervertebral implant according to claim 43, wherein the second piece snap fits into the first piece.
 45. An intervertebral implant according to claim 36, wherein the overall length of at least one single anchor in the direction of a path of insertion of the implant is greater than one half of the overall dimension of its respective part in the direction of insertion.
 46. An intervertebral implant according to claim 36, wherein the single anchor is a single uninterrupted solid piece extending outwardly from the outer surface of its respective part.
 47. An intervertebral implant according to claim 36, wherein the first and second parts are generally rectangular.
 48. An intervertebral implant according to claim 47, wherein the single anchor is generally parallel to the short sides of the generally rectangular shape. 